In the radiometric measurement of object points on the surface of an astronomical body conducted from platforms such as aircraft, spacecraft or satellites, the problem regularly posed is that of how to correct distortion in the image points of the object points which is caused, for example, by the structure and shape of the surface.
Of the many technical solutions for correcting distortion of the type mentioned above, only two will be mentioned here by way of example. WO 2007/130871 A2 discloses an optical adapter for use during satellite-based image acquisition by which distortion in an acquired image can be corrected subsequently and a distortion-free final image can be provided.
A correction of distortion based on the use of reference images is disclosed in DE 103 54 752 A1, wherein a mapping function is derived from a quantity of ground control points and is used for the correction. An approach of this kind, which aims at a posterior correction of image points, is commonly applied. The information content of individual, punctiform image points of object points is modified, e.g., averaged or converted. Renderings of the object point which are largely free of distortion can be achieved by means of corrections carried out subsequent to recording, but the “identity” of an object point, i.e., its qualitative characteristics which are actually measured, is lost.
The effect of imaging errors is especially detrimental when defined object points are measured successively in time by a plurality of detectors, e.g., linear-array detectors or area-array detectors, for what is known as co-registration. Imaging errors of this kind can be superposed one upon the other, particularly when measuring in equidistant time steps.
Therefore, DE 10 2008 030 727 A1 describes a method by which imaging errors occurring during the observation of astronomical bodies by satellite-based instruments are minimized already during image acquisition. The imaging errors are caused by the surface curvature and rotational movement of the astronomical body and/or the flight motion of the observing instrument. Minimization is carried out by means of sensor pixels of different sizes and by different combinations of a plurality of detector elements varied over time. In addition, the above-mentioned methods can be applied for reducing imaging errors.
A previously known high-performance camera HRSC (High Resolution Stereo Camera) described by R. Jaumann et al., DLR Nachrichten 116, 2006, 20-27, has been used in the Mars Express probe since 2004 for three-dimensional surveying of the surface of Mars. For this purpose, a probe orbits Mars and measures characteristics of object points by means of a measuring head having nine CCD lines, of which five are used for stereo recordings and photometric recordings and the other four for recording in different spectral regions. The detectors are arranged transverse to the flight direction and parallel to one another in the image plane of an objective. Therefore, each row acquires the object points with a time offset. Each detector line has approximately 5200 pixels (detector elements) by which an object point is acquired in each instance. This high-performance camera carries out very high-precision geometric and radiometric measurements. In so doing, however, it is not evident that every object point on the surface of the astronomical body is correctly associated spatially and temporally by the corresponding detector elements of each of the nine detector lines for the subsequent superposition of the matching pixel data (co-registration). However, the correct temporal and spatial superposition of the matching pixel data cannot be ensured for all pixels because of imaging errors resulting from the differing position of the detectors in the image plane of the objective.
Aside from the above-mentioned surface-dependent imaging errors, there also occur systematic imaging errors, i.e., those depending solely on the design layout of the technical system and the interaction thereof with the characteristics of the acquired electromagnetic waves. These imaging errors are relatively minor compared to imaging errors caused by surface curvature, but they still have a very detrimental influence on the achievable quality (precise pixel-to-pixel co-registration) of the measurements.